Effects of hydrophobic and superhydrophobic coatings of a condenser on the thermal performance of a two-phase closed thermosyphon

The effects of hydrophobic and superhydrophobic coatings in a condenser section on the thermal performance of a two-phase closed thermosyphon (TPCT) were experimentally investigated. The TPCT has an inner diameter of 25 mm and comprises a copper pipe and three sections, namely, an evaporator, an adiabatic section, and a condenser. Distilled water is used as a working fluid and the filling ratio of the TPCT is fixed at 0.5 based on the volume of the evaporator section. Its inclination angle varies from 15 degrees to 90 degrees (vertical), and its coolant temperature from 15 degrees C to 45 degrees C. The durable hydrophobic coating in the condenser section was formed through electrostatic spraying of perfluoroalkoxy alkane (PFA) resins, and the superhydrophobic coating was produced by adding a polytetrafluoroethylene layer on a nanostructured surface. The condensation heat transfer on the superhydrophobic coated surface showed an abrupt deterioration with an increasing heat flux, which was attributed to surface flooding observed inside the nanostructures for high supersaturation. On the hydrophobic coated surface, dropwise condensation was prolonged until a high heat flux was attained, and the transition to filmwise condensation was delayed. Therefore, the overall thermal resistance of the TPCT with the hydrophobic coating was lower than that with the bare surface by up to 45%, despite the additional thermal resistance through the PFA coating. (C) 2019 Elsevier Ltd. All rights reserved.